Gödel, Escher, Bach An Eternal Golden Braid by Douglas R. Hofstadter

Strange Loops, TNT, and Real Genetics

However, we have seen that in TNT, levels are mixed, in another sense. In
fact, the distinction between language and metalanguage breaks down:
statements about the system get mirrored inside the system. It turns out that
if we make a diagram showing the relationship between TNT and its
metalanguage, we will produce something which resembles in a remarkable
way the diagram which represents the Central Dogma of Molecular Biol-
ogy. In fact, it is our goal to make this comparison in detail; but to do so, we
need to indicate the places where Typogenetics and true genetics coincide,
and where they differ. Of course, real genetics is far more complex than
Typogenetics-but the "conceptual skeleton" which the reader has ac-
quired in understanding Typogenetics will be very useful as a guide in the
labyrinth of true genetics.

DNA and Nucleotides

We begin by discussing the relationship between "strands", and DNA. The

initials "DNA" stand for "deoxyribonucleic acid". The DNA of most cells

resides in the cell's nucleus, which is a small area protected by a membrane.

Gunther Stent has characterized the nucleus as the "throne room" of the

cell, with DN A acting as the ruler. DNA consists of long chains of relatively

simple molecules called nucleotides. Each nucleotide is made up of three

parts: (1) a phosphate group stripped of one special oxygen atom, whence

the prefix "deoxy"; (2) a sugar called "ribose", and (3) a base. It is the base

alone which distinguishes one nucleotide from another; thus it suffices to

specify its base to identify a nucleotide. The four types of bases which occur

in DN A nucleotides are:

A: adenine

G: guanine }

purines

C: cytosine

T: thymine }

pyrimidines

(Also see Fig. 91.) It is easy to remember which ones are pyrimidines

because the first vowel in "cytosine", "thymine", and "pyrimidine" is 'y'.

Later, when we talk about RNA, "uracil"-also a pyrimidine-will come in

and wreck the pattern, unfortunately. (Note: Letters representing nucleo-

tides in real genetics will not be in the Quadrata font, as they were in

Typogenetics.)

A single strand of DNA thus consists of many nucleotides strung

together like a chain of beads. The chemical bond which links a nucleotide

to its two neighbors is very strong; such bonds are called covalent bonds, and

the "chain of beads" is often called the covalent backbone of DNA.

Now DNA usually comes in double strands-that is, two single strands

which are paired up, nucleotide by nucleotide (see Fig. 92). It is the bases

(^514) Self-Ref and Self-Rep